Since the scotch-tape isolation of graphene, two-dimensional (2D) materials have been studied with increasing enthusiasm. Two-dimensional transition-metal dichalcogenides are of particular interest as atomically thin semiconductors. These materials are naturally transparent in their few-layer form, have direct band gaps in their monolayer form, exhibit extraordinary absorption, and demonstrate unique physics, making them promising for efficient and novel optical devices. Due to the two-dimensional nature of the materials, their properties are highly susceptible to the environment above and below the 2D films. It is critical to understand the influences of this environment on the properties of 2D materials and on the performance parameters of devices made with the materials. For transparent optical devices requiring electrical contacts and gates, the effect of transparent conducting oxides on the optical properties of 2D semiconductors is of particular importance. The ability to tune the optical properties of 2D transition-metal dichalcogenides could allow for improved control of the emission or absorption wavelength of optical devices made with the materials. Continuously tuning the optical properties of these materials would be advantageous for variable wavelength devices such as photodetectors or light emitters. This thesis systematically investigates the intrinsic structural and optical properties of two-dimensional transition-metal dichalcogenide films, the effect of substrate-based optical interference on the optical emission properties of the materials, and demonstrates methods to controllably tune the luminescence emission of the materials for future optical applications. This thesis advances the study of these materials toward integration in future efficient and novel optical devices. The specific transition metal dichalcogenides investigated here are molybdenum disulfide (MoS2), molybdenum diselenide (MoSe2), tungsten disulfide (WS2), and tungsten diselenide (WSe2). The thickness-dependence of the intrinsic in-plane crystal structure of these materials is elucidated with high-resolution transmission electron microscopy; thickness-dependent optical properties are studied using Raman and photoluminescence spectroscopies. This thesis investigates the optical interference effects from substrates with transparent conducting oxide layers on the optical properties of few-layer MoS2 films. An understanding of these effects is critical for integrating MoS2 into efficient optical devices. We predict contributions of optical interference effects to the luminescence emission of few-layer MoS2 films. The predictions are experimentally verified. We also demonstrate the use of optical interference effects to tune the wavelength and intensity of the luminescence emission of few-layer MoS2. This thesis explores the use of electric fields applied perpendicular to the films to continuously and reversibly tune the band gap of few-layer MoS2 for future variable wavelength devices. To facilitate integration into devices, we demonstrate electric fieldinduced band gap tuning by applying electric fields with a pair of transparent or semitransparent conducting layers, and without the need for direct electrical contact to the MoS2 films. The observed band gap tuning is attributed to the Stark Effect. We discuss challenges to maximizing the effect of electric field-induced band gap tuning. We demonstrate that optical interference effects do not prevent observation of band gap tuning via applied electric fields. We successfully combine two luminescence emission tuning methods: optical interference effects and electric field effects.

van der Waals heterostructures meet other low-dimensional materials. Stacking of about 1 nm thick nanosheets with out-of-plane anchor groups functionalized with fullerenes integrates this zero-dimensional material into layered heterostructures with a well-defined chemical composition and without degrading the mechanical properties. The developed modular and highly applicable approach enables the incorporation of other low-dimensional materials, e.g. nanoparticles or nanotubes, into heterostructures significantly extending the possible building blocks.

Gaseous pollution has become a global issue and its presence above certain limits is hazardous to human health and environment. Detection of such gases is an immediate need and researchers around the world are trying to solve this problem. Metal oxides are being used as sensing materials for a long time, but a high operating temperature limits applications in many areas. On the other hand, two-dimensional (2D) materials with high surface-to-volume ratio and chemical stability are promising candidates in the field of gas sensing. This includes monolayer transition metal dichalcogenides, such as MoS2 and WS2, which are direct band gap materials. While few layer transition metal dichalcogenides are indirect band gap materials, they are easier to synthesize than monolayers. Therefore, it is important to understand whether few layer transition metal dichalcogenides possess the same sensing behavior as the corresponding monolayers. For this reason the first part of this dissertation compares the sensing behavior of monolayer and few layer MoS2 and WS2. Two dimensional hexagonal boron nitride is a highly stable structural analogue of graphene. However, its insulating behavior with large band gap is not suitable for sensing. Recently, monolayer Si2BN has been proposed to exist. As the presence of Si makes this material reactive, the second part of this dissertation addresses its application as sensing material. In the _nal part of this dissertation, in search of a metal free, non-toxic, and earth abundant sensor material, further structural analogues of graphene are considered, namely monolayer C3N, monolayer C3Si, and monolayer C6BN. In particular, different theoretical approaches for studying the sensing performance of materials are compared to each other.

Polymer brushes can be used to tailor the physical and chemical properties of materials on demand to meet potential applications. Therefore, fabrication of polymer brushes with well-defined structure and functional groups enables the engineering of new materials with diverse functions. In addition, two-dimensional (2D) materials have their unique physical/chemical properties and potential applications in (opt)electronics, catalysis, energy storage, sensing, and other related fields. However, the dispersibility, chemical stability, charge transport behavior, mechanical properties of the 2D materials hinder their further applications. Therefore, combining polymer brushes and 2D materials may bring in new properties which are not available by either of them alone. This thesis focuses on brushing up 2D materials (from inorganic to organic) with universal photografting techniques. (1) The first chapter introduces the outline and research content of the thesis. (2) The second chapter describes the background of 2D materials and polymer brushes. In particular, this chapter analyzes mechanisms, drawbacks and benefits of different polymerization methods, and also summarizes the general approaches to grow polymer brushes on 2D material surfaces, coupling with potential applications of polymer functionalized 2D materials. (3) The third chapter shows the motivation and aim of this thesis. (4) The fourth chapter discusses the results of the functionalization of hexagonal boron nitride (hBN), MoS2, graphitic-carbon nitride (gCN), alkyl-polydopamine (alkyl-PDA), and conjugated 2D polymers (2DPs) with polymer brushes by the same self-initiated photografting and photopolymerization (SIPGP) method and their related applications in detail, respectively. First, the direct photopolymerization of vinyl monomers results in the formation of thick and homogeneous polymer brushes covalently bounded to hBN. The brush layer mechanically and chemically stabilizes the material and allows facile handling as well as long-term use in water splitting hydrogen evolution reactions. Second, the chapter demonstrates the MoS2 can be directly modified with polymer brushes by SIPGP. After modifying MoS2 with polymer brushes, the dispersibility of polymer brushes-modified MoS2 was obviously improved. Third, the polymer brushes functionalized gCN significantly improves the dispersibility. Application of polymer brush functionalized gCN as excellent recyclable substrates for an outstanding SERS as well as photocatalytic degradation of dyes is demonstrated. Fourth, to directly obtain the 2D materials with functional groups, the chapter proposes a facile method to prepare amphiphilic polymeric Janus nanosheets with hydrophilic PDA and hydrophobic alkyl chains at both sides. Benefiting from the Janus property of the alkyl-PDA nanosheets, the nanosheets can be grafted polymer brushes through photografting and be conjugated Fe3O4 nanoparticles selectively onto the PDA side. Finally, the chapter shows that various polymer brushes can be directly grafted onto 2DPs and freestanding system is also obtained. Moreover, it is found that the morphology of freestanding system quickly and reversibly responds to solvent quality by shrinking/stretching. (5) The fifth chapter addresses the general conclusion and future prospective of the whole work. (6) The sixth chapter describes the experiment part of the whole thesis.

Since the first successful isolation and subsequent characterization of graphene, the interest in two dimensional (2-D) materials has expanded exponentially. Despite the dozens of graphene-like van der Waals materials that have been found and their interesting properties, a significant obstacle in realizing their promise is their instability especially for monolayer and thin layers at elevated temperature. To overcome the obstacle of passivating the 2-D materials and study their properties at elevated temperature, we take advantage of the potential improvements afforded by assembling heterostructures by stacking the atomic thick 2-D materials together hexagonal boron nitride (ℎ-BN) which possess high chemical stability and thermal stability. In this dissertation, several experiments are described in detail in which we utilized h-BN encapsulation to passivate atomically-thin transition metal dichalcogenide and studied their properties at elevated temperature. In the first project we demonstrated that chemical vapor deposition (CVD)-grown flakes of high-quality monolayers of WS₂ can be stabilized at elevated temperatures by encapsulation with only top ℎ-BN layers in the presence of ambient air, N₂ or forming gas. The best passivation occurs for ℎ-BN covered samples with flowing N₂. In the second project, we demonstrated that encapsulating monolayer MoSe₂ and WS₂ with top and bottom ℎ-BN can improve their thermal stability at high temperature and increase their photoluminescence (PL). The increased PL likely occurs because impurities are laterally expelled from the TMD stack during heating. In the third project, we demonstrated the passivation of different modes of ℎ-BN encapsulation on thin layer FeSe sample by using temperature dependent Raman scattering. The complete encapsulation showed the best protection of thin layer FeSe. Finally, we utilized the temperature dependence of the Raman mode of thin-layer FeSe with complete encapsulation and applied a noncontact method to measure the thermal conductivity of the thin-layer FeSe.

This thesis investigates the unique properties of 2D devices such as twisted semiconducting bilayers and a-RuCl₃ heterostructures employing scanning tunneling microscopy (STM) and spectroscopy (STS) probes. The research presented here sheds light on the vast opportunities that 2D materials provide in condensed matter systems as well as future device applications. Among 2D materials, transition metal dichalcogenide (TMD) heterobilayers provide a promising platform to study many quantum phenomena such as excitonic states due to their tunability of band gap. In addition, TMDs are excellent candidates to achieve localized states and carrier confinement, crucial for single photon emitters used in quantum computation and information. We begin this thesis with a brief overview of STM/STS and utilizing these techniques on 2D materials in the first and second chapters.The third chapter of this work investigates the twisted bilayer of WSe₂ and MoSe₂ in the H-stacking configuration using STM/STS which was previously challenging to measure. The spectroscopic results obtained from the heterobilayer indicate that a combination of structural rippling and electronic coupling generates unexpectedly large \moire potentials, in the range of several hundred meV. Our analysis reveals that the \moire structure and internal strain, rather than interlayer coupling, are the main factors of the moire potential. Large moire potentials lead to deeply trapped carriers such as electron-hole pairs, so-called excitons. Our findings open new routes toward investigating excitonic states in twisted TMDs. In the next chapter, we investigate the ultralocalized states of twisted WSe₂/MoSe₂ nanobubbles. Mechanical and electrical nanostructurings are expected to modify the band properties of transition metal dichalcogenides at the nanoscale. To visualize this effect, we use STM and near-field photoluminescence to examine the electronic and optical properties of nanobubbles in the semiconducting heterostructures. Our findings reveal a significant change in the local bandgap at the nanobubble, with a continuous evolution towards the edge of the bubble. Moreover, at the edge of the nanobubble, we show the formation of in gap bound states. A continuous redshift of the interlayer exciton on entering the bubble is also detected by the nano-PL. Using self-consistent Schrodinger-Poisson simulations, we further show that strong doping in the bubble region leading to band bending is responsible for achieving ultralocalized states. Overall, this work demonstrates the potential of 2D TMDs for developing well-controlled optical emitters for quantum technologies and photonics. We next turn to the effect of the electric field in band gap tuning of WSe₂/WS₂ heterobilayer. The tunability of band gap is a crucial element in device engineering to achieve quantum emitters. The electrostatic gate generates doping and an electric field giving access to continuous tunability, higher doping level, and integration capability to nanoelectronic devices. We employ scanning tunneling microscopy (STM) and spectroscopy (STS) to probe the band properties of twisted heterobilayer with high energy and spatial resolution. We observe continuous band gap tuning up to several hundreds of meV change by sweeping the back gate. We introduced a capacitance model to take into account the finite tip size leading to an enhanced electric field. The result of our calculation captures well the band gap change observed by STS measurements. Our study offers a new route toward creating highly tunable semiconductors for carrier confinement in quantum technology. In the next chapters, we focus on a-RuCl₃ heterointerfaces. We first explore the nanobubble of graphene/a-RuCl3 to create sharp p-n junctions. The ability to create sharp lateral p-n junctions is a critical requirement for the observation of numerous quantum phenomena. To accomplish this, we used a charge-transfer based heterostructure consisting of graphene and a-RuCl₃ to create nanoscale lateral p-n junctions in the vicinity of nanobubbles. Our approach relied on a combination of scanning tunneling microscopy (STM) and spectroscopy (STS), as well as scattering-type scanning near-field optical microscopy (s-SNOM), which allowed us to examine both the electronic and optical responses of these nanobubble p-n junctions. Our results showed a massive doping variation across the nanobubble with a band offset of 0.6 eV. Further, we observe the formation of an abrupt junction along nanobubble boundaries with an exceptionally sharp lateral width (<3 nm). This is one order of magnitude smaller length scale than previous lithographic methods. Our work paves the way toward device engineering via interfacial charge transfer in graphene and other low-density 2D materials. In chapter 7, we describe the use of low-temperature scanning tunneling microscopy (STM) measurements to observe the \moire pattern in graphene/a-RuCl3 heterostructure to validate the InterMatch method. This method is effective in predicting the charge transfer, strain, and stability of an interface. The InterMatch method was applied to moire patterns of graphene/a-RuCl3 to predict the stable interface structure. STM topographs show three regions with distinct moire wavelengths due to atomic reconstructions. Using the InterMatch method, we perform a comprehensive mapping of the space of superlattice configurations and we identify the energetically favorable superlattices that occur in a small range of twist angles. This range is consistent with the STM results. Moreover, the spectra on these regions exhibit strong resonances with the spacing between resonances following the expectation from Landau levels on a Dirac spectrum due to strain and doping. The results of our scanning tunneling microscopy (STM) measurements confirm that the InterMatch method is effective in predicting the charge transfer and stability of interfaces between materials. We next investigate WSe₂/a-RuCl₃ heterostructure through a multi-faceted approach. Our exploration encompassed diverse techniques such as STM, and optical measurements. We detect a significant charge transfer between the two layers by STM measurements, leading to a shift in the Fermi level towards the valence band of WSe₂. Our findings are supported by optical measurements and DFT calculations, which confirm the p-doped WSe₂ observed through STM. The results of this work highlight a-RuCl₃ potential for contact engineering of TMDs and unlocking their functionalities for the next generation optoelectronic devices. In the last chapter of this thesis, I provide a brief conclusion as well as a few future directions and insights for investigating 2D materials.

Thesis advisor: Kevin S. Bedell / We present several results based upon non-trivial extensions of Landau-Fermi liquid theory. First proposed in the mid-20th century, the Fermi liquid approach assumes an adiabatic “switching-on” of the interaction, which allows one to describe the collective excitations of the many-body system in terms of weakly-interacting quasiparticles and quasiholes. At its core, Landau-Fermi liquid theory is often considered a perturbative approach to study the equilibrium thermodynamics and out-of-equilibrium response of weakly-correlated itinerant fermions, and therefore non-trivial extensions and consequences are usually overlooked in the contemporary literature. Instead, more emphasis is often placed on the breakdown of Fermi liquid theory, either due to strong correlations, quantum critical fluctuations, or dimensional constraints. After a brief introduction to the theory of a Fermi liquid, I will first apply the Landau quasiparticle paradigm to the theory of itinerant Majorana-like fermions. Defined as fermionic particles which are their own anti-particle, traditional Majorana zero modes found in topological materials lack a coherent number operator, and therefore do not support a Fermi liquid-like ground state. To remedy this, we will apply a combinatorical approach to build a statistical theory of self-conjugate particles, explicitly showing that, under this definition, a filled Fermi surface exists at zero temperature. Landau-Fermi liquid theory is then used to describe the interacting phase of these Majorana particles, from which we find unique signatures of zero sound in addition to exotic, non-analytic contributions to the specific heat. The latter is then exploited as a “smoking-gun” signature for Majorana-like excitations in the candidate Kitaev material Ag3LiIr2O6, where experimental measurements show good agreement with a sharply-defined, “Majorana-Fermi surface” predicted in the underlying combinatorial treatment. I will then depart from Fermi liquid theory proper to tackle the necessary conditions for the applicability of Luttinger’s theorem. In a nutshell, Luttinger’s theorem is a powerful theorem which states that the volume of phase space contained in the Fermi surface is invariant with respect to interaction strength. In this way, whereas Fermi liquid only describes fermionic excitations near the Fermi surface, Luttinger’s theorem describes the fermionic degrees of freedom throughout the entire Fermi sphere. We will show that Luttinger’s theorem remains valid only for certain frequency and momentum-dependencies of the self-energy, which correlate to the exis- tence of a generalized Fermi surface. In addition, we will show that the existence of a power-law Green’s function (a unique feature of “un-particle” systems and a proposed characteristic of the pseudo-gap phase of the cuprate superconductors) forces Luttinger’s theorem and Fermi liquid theory to be mutually exclusive for any non-trivial power of the Feynman propagator. Finally, we will return to Landau-Fermi liquid theory, and close with novel out-of-equilibrium behavior and stability in unconventional Fermi liquids. First, we will consider a perfectly two- dimensional Fermi liquid. Due to the reduction in dimension, the traditional mode expansion in terms of Legendre polynomials is modified to an expansion in terms of Chebyshev polynomials. The resulting orthogonality conditions greatly modifies the stability and collective modes in the 2D system. Second, we will look at a Fermi liquid in the presence of a non-trivial gauge field. The existence of a gauge field will effectively shift the Fermi surface in momentum space, resulting in, once again, a modified stability condition for the underlying Fermi liquid. Supplemented with a modernized version of Mermin’s condition for the propagation of zero sound, we outline the full effects a spin symmetric or anti-symmetric gauge would have on a Fermi liquid ground state. / Thesis (PhD) — Boston College, 2021. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Physics.

Silicon photonics has been widely recognized as a key technology that enables guiding, modulating, detecting, and computing of light in silicon chips. Photonic chips can be fabricated in a similar fashion as microelectronic chips, leveraging the mature CMOS fabrication and metrology infrastructure. Extending this technology, this dissertation focuses on two different areas : silicon microresonator-based mid-infrared light sources, and efficient strain engineering of the bandgap of two-dimensional materials. First, we review the basic theory of waveguides and ring resonators, laying the groundwork for the rest of the dissertation. Second, nonlinear optics is introduced with an emphasis on third order nonlinear phenomena including four wave mixing, the basis for Kerr frequency comb generation. Third, starting with the basic theory of lasers, we present the basic principles of quantum well lasers, leading to the discussion of quantum and interband cascade lasers. Fourth, we demonstrate a simple approach to generate mid-infrared frequency comb using a passive high-Q microresonator as well as an over one million quality factor silicon microresonator at 4.5 ?m. The novel suspended inverse taper with sub-3dB coupling loss is reported. Fifth, we demonstrate a compact narrow-linewidth widely-tunable mid-infrared laser using a high-Q external on-chip cavity. Lastly, we demonstrate highly efficient modulation of transition metal dichalcogenide monolayers (TMD) monolayers as well as TMD monolayer integrated on a silicon nitride waveguide. Additionally, we present a heterogeneous integration platform based on a thin polymer, which allows bonding as well as in principle, evanescent coupling between the two substrates.

Charge transfer at the organic/inorganic semiconductor interfaces lies at the heart of interfacial photochemistry. While decades of research have shaped the current understanding that interfacial charge transfer depends crucially on energetic driving force and electronic coupling, much less is known about the role played by the spin degree of freedom. In particular, it is not clear how spin states evolve during the charge transfer process. With the advent of group 6 transition metal dichalcogenides (TMDC), a class of two-dimensional layered materials which permits the optical generation of spin-polarized electron-hole pairs in the monolayer limit, we now have the opportunity to investigate if charge transfer at an organic/inorganic interface could enable the transfer of spin polarization. Using time-resolved Faraday rotation and transient absorption spectroscopy, it is found in the MoSe₂/H₂Pc and C60/WS₂ heterostructures that the photoinduced hole transfer from MoSe₂ to H₂Pc and electron transfer from WS₂ to C60 results in spin polarization lifetimes one order of magnitude longer than that of a monolayer. In the WS₂/MoSe₂/H₂Pc heterostructure, the addition of a WS₂ monolayer drives the dissociation of electron-hole pairs bound at the MoSe₂/H₂Pc interface and leads to the observation of nanosecond-long spin polarization at room temperature. These findings evidence the photoinduced transfer of spin polarization, a mechanism which could potentially be exploited to enhance the efficiency and selectivity of photochemical reactions involving angular momentum change, and may be generalized to other organic/inorganic interfaces composed of crystalline semiconductors with spin-momentum locking.

L'oxyde de silicium est un composé très largement abondant qui existe sous différentes phases, cristallines ou amorphes, qui se présentent sous la forme de structures poreuses ou de films minces. Il s'agit d'un diélectrique traditionnel pour la microélectronique et d'un support de choix pour des nanoparticules dans des systèmes catalytiques. Sa structure, amorphe ou tridimensionnelle et complexe, rend difficile la compréhension des propriétés jusqu'aux échelles les plus élémentaires. Les films utra-minces épitaxiés, parfois nommés « silice bidimensionelle » se prêtent au contraire à des caractérisation fines de la structure et des propriétés.Cette thèse avait pour objectif de préparer une telle phase d'oxide de silicium. A l'aide de sondes de sciences des surfaces, la microscopie à effet tunnel (STM), la diffraction d'électrons rapides en réflexion (RHEED), dont les analyses ont été confrontées aux résultats de calculs en théorie de la fonctionnelle de la densité (DFT), la structure de cette phase à pu être résolue jusqu'à l'échelle atomique. Nous avons mis en évidence l'arrangement hexagonal de tétraèdres de [SiO4], chimisorbés sur la surface (0001) du ruthenium en des sites spécifiques. Une phase d'oxygène diluée, reconstruite sur le Ru(0001), a été observée, qui coexiste avec l'oxide de silicium.La croissance de l'oxyde de silicium, a également été étudiée, par un suivi in operando, en temps réel pendant la croissance, par RHEED. Une évolution marquée de taille de domaines et/ou de l'accumulation et de la relaxation de déformations a été observée alors que l'oxyde de silicium crystallise. Un mécanisme de croissance a été proposé sur la base de ces observations, selon lequel les espèces chimiques à la surface se réorganisent par des déplacements latéraux élémentaires. Ce mécanisme s'accompagne de la formation, inévitable, de lignes de défauts uni-dimensionnelles, dont la structure a été déterminée à l'échelle atomique par STM. / Silicon oxide is a widely abundant compound existing in various forms from amorphous to crystalline, bulk to porous and thin films. It is a common dielectric in microelectronics and widely used host for nanoparticles in heterogenous catalysis. Its amorphous nature and the ill-defined complex three dimensional structure is a hurdle to the understanding of its properties down to the smallest scales. Resorting to epitaxially grown ultra-thin phase (also called a two-dimensional material) can help overcome these issues and provide clear-cut information regarding the structure and properties of the material.In this thesis, studies were aimed at growing this promising novel phase of silicon oxide. Using surface science tools, including scanning tunelling microscopy (STM) and reflection high energy electron diffraction (RHEED) supported by density functional theory calculations, the atomic structure was resolved to high resolution. The monolayer was found to have a hexagonal arrangement of the [SiO4] tetrahedra chemisorbed on the surface of Ru(0001) into specific sites. This lattice of monolayer silicon oxide was also found to coexist with an oxygen reconstruction of the bare Ru(0001) inside each silicon oxide cell.The growth of this monolayer was monitored in real-time by in operando RHEED studies. These experiments provided with insights the domain size evolution and the build up/release of strain field during the growth that. Based on the experimental observations, a growth mechanism leading to the formation of monolayer silicon oxide could be proposed in terms of geometrical translations of the atomic species on the surface of Ru(0001) support. This mechanism results in unavoidable formation of one-dimensional line-defects that were precisely resolved by the STM.